This invention relates to a novel method of preparing a silver catalyst and its use in the process of making ethylene oxide by the partial oxidation of ethylene in the vapor phase. Silver-containing catalysts in which the catalytically active component is the metal itself are well known in the art. An important use for the catalyst is in the direct oxidation conversion of alkenes to the corresponding vicinal epoxides, particularly in preparing ethylene oxide from ethylene by reacting ethylene with oxygen in the vapor phase.
Methods known to the art for making such catalysts include soaking a carrier or support in aqueous solutions of silver salts to impregnate it. Thereafter the thus-impregnated salts are reduced to silver metal prior to utilization in the process for oxidizing ethylene. Reduction is normally accomplished by heating in the presence of a reducing agent or by thermal decomposition of the salt. This is done at temperatures within the range of 125.degree. C. to 400.degree. C. and preferably from 200.degree. C. to 300.degree. C. Alternatively, the silver salt may be deposited from a slurry. Either slurry or solution also may contain a reducing agent, or the reducing agent may be subsequently applied.
The commonly used reducing agents are organic compounds which include polyhydric alcohols, such as liquid glycols (e.g. ethylene, propylene, and butylene glycols), glycerol, aqueous sugar solutions, aqueous polyvinyl alcohol solutions, the polyglycols, (e.g. polyethylene and polypropylene glycols) preferably of relatively low molecular weight; also included are aqueous solutions of such polyglycols, the water soluble glycol alkyl ethers, and the like. Other excellent reducing agents are high-boiling esters of carboxylic acids such as dioctyl sebacate, dibutyl phthalate, and the like.
One of the criteria for commercially useful silver catalysts is that the silver be finely divided and relatively homogeneously dispersed on the catalyst support. Dispersing agents are advantageously used in order to obtain such silver deposits, especially suitable as dispersing agents are organic amines such as ethylene diamine and ethanolamine and others disclosed in U.S. Pat. No. 3,702,259; and those naturally occurring gums such as disclosed in U.S. Pat. No. 3,887,491. These natural gums are, for example, karaya, ghatti, and tragacanth, which are plant exudates; root or seed extracts, such as guar, saponin and locust bean, psyllium seed, and quince seed. Seaweed extracts such as agar, carrageenin and furcellaran are also useful as well as others such as gelatin, casein, and pectin. Certain chemically modified derivatives of starch or cellulose and poly sacharides (the unmodified forms are which are insoluble) are also included as substances classifiable as gums and are useful as dispersing agents in the preparation of catalysts.
The condition of silver, that is the state of subdivision, is of great importance in the successful, efficient utilization of these catalysts for ethylene oxidation to ethylene oxide. This is recognized in U.S. Pat. No. 3,043,854 wherein a process for making catalysts having silver particles of less than 1.mu. is described. The importance of uniformity of size is also recognized in U.S. Pat. No. 3,702,259 wherein small, uniform-sized particles are produced by the use of a solubilizing-reducing agent such as, for example, ethylene diamine or ethanolamine or their mixtures.
Supports known to be useful for making silver catalysts are for example alumina, zirconia, corundum, mullite, silicon carbide and carbon. Alumina is preferred and especially a porous alumina of low surface area, i.e. less than one square meter per gram.
While silver is the metal most useful from a commercial standpoint in providing the catalytic effect necessary to obtain ethylene oxide, most commercial catalysts additionally contain small amounts of a promoter. The amount employed is usually from a few parts per million up to one or two percent, based on the weight of the total catalyst. Representative promoters include the alkali and alkaline earth metals which are usually present as their oxides. Thus lithium, sodium, potassium, rubidium, cesium, calcium, barium, cadmium, and the like, are added as their salts to the solution of the silver salt which is applied to the support and on subsequent heating are converted to their oxides.
Other ways known to the art of adding the promoter compound are to add it to the support prior to (see U.S. Pat. No. 3,563,914) or subsequent to (see U.S. Pat. Nos. 2,142,948 and 2,404,438) the application of the silver salt. In each case the particular salt applied is dried prior to applying the solution of the second salt. In a more recent patent (U.S. Pat. No. 4,168,247), the promoters are applied after reduction of the salt to the silver metal coating, but then the entire catalyst is heat treated. To insure adequate penetration of the pores of the support, a vacuum is applied when applying the aqueous solutions of the silver salt or of the promoter salt. This is described in U.S. Pat. No. 3,575,888. Generally the promoters are converted to their oxides and the silver salt is reduced to silver.
A heat treatment of the catalyst prior to applying cesium as a promoter is taught in U.S. Pat. No. 4,033,903. This heat treatment is in addition to the heating employed to (1) reduce the silver compound on the support to silver metal and (2) remove any reducing agent remaining on the catalyst. According to this patent one can heat-treat a new catalyst before applying the cesium, or apply cesium to one which has been employed as a catalyst in a reactor for the production of ethylene oxide, i.e., heat-treated by use. It is taught that such heat-treatment produces optimum stabilized particles having average diameters between 0.2 and 4.mu. and especially of between 0.4 and 2.5.mu..
It has now been discovered that the silver catalysts prepared by the methods disclosed in U.S. Pat. No. 3,887,491 and in copending application U.S. Ser. No. 043,414 produce a catalyst having a predominance of silver particles within the range of optimum stabilized particles taught in U.S. Pat. No. 4,033,903. While the range taught in this patent is from 0.2 to 4.mu., most of the particles produced by the methods of U.S. Pat. No. 3,887,491 and the above copending application are equal to or less than 1.mu., predominantly in the range of from 0.2 to 0.6.mu. in diameter. Such silver particles are especially suitable for the application of cesium as a promoter. It is not required that the silver catalyst be heat-treated to obtain the desired particle size for the silver prior to the application of the cesium as taught in U.S. Pat. No. 4,033,903 or thereafter as in U.S. Pat. No. 4,168,247, both noted above.